The unitary, rigid, self-supporting, gas permeable, low temperature bonded, sand grain molds described in that patent and application have, over the past several years, succeeded in replacing the more expensive high temperature resistant ceramic molds in many applications. However, in certain applications, such as those in which the lower end of the mold cavity extends for a considerable distance below the surface of the molten metal which is drawn up through the gate passage and into the mold cavity by application of reduced pressure to the upper surface of the mold, gas defects, resulting in ruined cast parts, have at times been a problem. In addition, the thickness of the cast parts had sometimes been limited by the appearance of gas defects as the thickness of the parts increased.
1. Field of the Invention
This invention relates to metal casting in unitary, rigid, self-supporting, gas permeable, low temperature bonded, sand grain molds having one or more mold cavities adapted to be filled with molten metal by applying reduced pressure to the top surface of the mold while the lower surface of the mold with the mold cavity gate passage open ends is submerged in molten metal. More particularly, it relates to improvements in such molds which makes it possible to cast in such molds certain types of parts, particularly those having deep mold cavities and those of increased wall thickness, free from the gas defects which otherwise would be expected.
2. Description of the Prior Art
The present invention, similarly to the inventions of the patent and application, is concerned only with molds of the low temperature bonded, sand grain type, which "self-destruct" in the presence of molten ferrous metals in a matter of seconds. Such molds have been used for decades in gravity casting, although in a much different construction in which the gate passages extended along the mold parting plane from the mold cavities and terminated with their open ends at the side or top of of the mold in the mold parting plane.
However, until the inventions of the patent and application, which require a unique mold construction in which at least portions of the gate passages extend perpendicularly to the mold parting plane and terminate with their open ends at the lower mold surface, such low temperature bonded, sand grain molds had never been successfully used in vacuum casting, although their cost advantages over high temperature ceramic molds had long been known. The prior art, then, is properly limited to such molds.